(132g) Conversion of C6 Sugars to Alpha-Hydroxy Acids over Lewis Acidic Hf-, Sn-, and Zr- Beta Zeolite Catalysts Using ?-Valerolactone As Solvent
The production of biodegradable polymers such as polylactic acid (PLA) has emerged as an area of significant interest based on the recent development of catalytic processes for production of the monomeric building block, lactic acid (LA), from renewable feedstocks. However, these processes are limited to the use of an alcohol, such as methanol, as a solvent to obtain high yields of intermediate LA derivatives (i.e., methyl lactate) due to low LA selectivity and activity obtained in the presence of water.Â Recently, polar aprotic organic solvents such asÂ Î³-valerolactone (GVL) have been shown to cause significant increases in reaction rates in addition to increased product selectivity for BrÃ¸nsted acid-catalyzed reactions compared to water . In this work, we investigated a similar effect for the Lewis acid-catalyzed conversion of glucose and fructose to lactic acid and other alpha-hydroxy acids (AHA) using various Lewis acid beta zeolites. Here we demonstrate that the combination of Beta zeolites with mainly Lewis acidity prepared using a post-synthetic procedure to incorporate tin, zirconium or hafnium in the framework of the zeolite and a solution of GVL and water as reaction medium is an effective catalytic process for the production of lactic acid and other AHAs from glucose and fructose. Furthermore, we address the separation of the AHAs from GVL by adding an organic co-solvent like toluene to the reaction medium that, at room temperature, forms a bi-phasic solvent system allowing the AHAs to concentrate in the aqueous phase . Characterization of the beta zeolite modified materials by nitrogen adsorption, X-ray Diffraction, X-ray Photoelectron Spectroscopy, Fourier Transform Infrared Spectroscopy and Inductively Coupled Plasma Atomic Emission Spectroscopy demonstrate that highly crystalline beta zeolite catalysts were synthesized with Al content as low as 236 ppm. We assessed the effect of the solvent and metal loading in the beta zeolite on the catalytic performance for the conversion of C6 sugars to LA and alpha hydroxyl-acids. The results demonstrate that as the GVL to water ratio is increased, there is an increase in the rates of glucose and fructose isomerization as well as the rate for retro-aldol condensation and the rates of AHAs production while at the same time, decreasing the formation of degradation products. In addition, we demonstrate that LA and AHAs have a higher affinity to the aqueous phase compared to the organic-rich phase, therefore, efficient separation of desired products from the GVL is obtained.